Don't print the default of string if it's NULL

Usually you would make the default of the string NULL only if you want
to make it mandatory. In that case there is not much need to display the
default at all, 'cause there is no default.

Author: rexim

flag.h

@@ -262,7 +262,9 @@ void flag_print_options(FILE *stream)
             fprintf(stream, "        Default: %" PRIu64 "\n", flag->def.as_uint64);
             break;
         case FLAG_STR:
-            fprintf(stream, "        Default: %s\n", flag->def.as_str);
+            if (flag->def.as_str) {
+                fprintf(stream, "        Default: %s\n", flag->def.as_str);
+            }
             break;
         default:
             assert(0 && "unreachable");